This invention relates in general to a hydraulic power output unit for generating hydraulic power on a vehicle. In particular, this invention relates to an improved structure for such a hydraulic power output unit and to the use of such an improved hydraulic power output unit in a hydraulic hybrid drive system for propelling a vehicle.
Drive train systems are widely used for generating power from a source and for transferring such power from the source to a driven mechanism. Frequently, the source generates rotational power, and such rotational power is transferred from the source of rotational power to a rotatably driven mechanism. For example, in most land vehicles in use today, an engine generates rotational power, and such rotational power is transferred from an output shaft of the engine through a driveshaft to an input shaft of an axle assembly so as to rotatably drive the wheels of the vehicle.
In some of these land vehicles and other mechanisms, a hybrid drive system (also known as an energy recovery system) is provided in conjunction with the drive train system to decelerate the rotatably driven mechanism, accumulate the energy resulting from such deceleration, and use the accumulated energy to subsequently accelerate the rotatably driven mechanism. To accomplish this, a typical hybrid drive system includes a reversible energy transfer machine that is coupled to the drive train system and an energy storage device that communicates with the reversible energy transfer machine. To decelerate the vehicle, the hybrid drive system is operated in a retarding mode, wherein the reversible energy transfer machine slows the rotation of the rotatably driven mechanism and stores the kinetic energy of the vehicle in the energy storage device as potential energy. To subsequently accelerate the vehicle, the hybrid drive system is operated in a driving mode, wherein the potential energy stored in the energy storage device is supplied to the reversible energy transfer machine to rotatably drive the rotatably driven mechanism. In a hydraulic type of hybrid drive system, pressurized fluid is used as the actuating mechanism. In such a hydraulic hybrid drive system, an accumulator functions as the energy storage device, and one or more hydraulic pump/motors function as reversible hydraulic machines.
It is known to provide certain vehicles with hydraulic power output units for providing hydraulic power to operate certain vehicle functions or accessories. One example of a hydraulic accessory that is driven by a hydraulic power output unit is a trash compacting cylinder on a refuse truck. The hydraulic power output unit is often embodied as a hydraulic pump that is selectively connectable to the vehicle transmission through a conventional power take-off.
When the vehicle is equipped with a hydraulic hybrid drive train system, a hydraulic power output unit provides hydraulic power to drive the driven wheels of the vehicle. There are generally two types of hydraulic hybrid drive train systems. In a parallel hybrid hydraulic drive train system, the vehicle includes both a conventional transmission as well as a hydraulic drive train system. A series hybrid hydraulic drive train system includes only the hydraulic drive train system, and the conventional transmission is removed from the vehicle. In both the parallel and series hybrid hydraulic drive train systems, the hydraulic power output unit forms a portion of the hydraulic drive train system for providing hydraulic power to a hydraulic motor for driving the driven wheels of the vehicle.
One problem encountered in vehicle drive train assemblies and other rotatable structures is that they tend to vibrate during operation. It is known that all mechanical bodies have a natural resonant frequency at which they tend to vibrate when operated at certain rotational speeds. This natural resonant frequency is an inherent characteristic of the mechanical body and is based upon many factors, including its composition, size, and shape. In the context of vehicular drive train assemblies, the engine and transmission assembly can sometimes generate vibrations that are transmitted to and accentuated by the driveshaft tube when rotated. Also, driveshaft tube may itself be rotated at a velocity that is at or near its natural resonant frequency (or one or more of the harmonics thereof), causing vibrations to be induced therein. In either event, the vibrations generated in the driveshaft tube are usually considered to be undesirable. Thus, it would be desirable to provide an improved structure for a hydraulic power output unit that minimizes this problem.